Principal part

The principal part at ${\displaystyle z=a}$ of a function

${\displaystyle f(z)=\sum _{k=-\infty }^{\infty }a_{k}(z-a)^{k}}$

is the portion of the Laurent series consisting of terms with negative degree.[1] That is,

${\displaystyle \sum _{k=1}^{\infty }a_{-k}(z-a)^{-k}}$

is the principal part of ${\displaystyle f}$ at ${\displaystyle a}$. If the Laurent series has an inner radius of convergence of 0 , then ${\displaystyle f(z)}$ has an essential singularity at ${\displaystyle a}$, if and only if the principal part is an infinite sum. If the inner radius of convergence is not 0, then ${\displaystyle f(z)}$ may be regular at ${\displaystyle a}$ despite the Laurent series having an infinite principal part.

• Mittag-Leffler's theorem
• Cauchy principal value

• Cauchy Principal Part at PlanetMath